Human beings have just two ears, but can locate sounds in three dimensions, in distance and in direction. This is possible because the brain, the inner ears, and the external ears (pinna) work together to make inferences about the location of a sound. The location of a sound is estimated by taking cues derived from one ear (monoaural cues), as well as by comparing the difference between the cues received in both ears (binaural cues).
Binaural cues relate to the differences of arrival and intensity of the sound between the two ears, which assist with the relative localization of a sound source. Monoaural cues relate to the interaction between the sound source and the human anatomy, in which the original sound is modified by the external ear before it enters the ear canal for processing by the auditory system. The modifications encode the source location relative to the ear location and are known as head-related transfer functions (HRTF).
In other words, HRTFs describe the filtering of a sound source before it is perceived at the left and right ear drums, in order to characterize how a particular ear receives sound from a particular point in space. These modifications may include the shape of the listener's ear, the shape of the listener's head and body, the acoustical characteristics of the space in which the sound is played, and so forth. All these characteristics together influence how a listener can accurately tell what direction a sound is coming from. Thus, a pair of HRTFs accounting for all these characteristics, generated by the two ears, can be used to synthesize a binaural sound and accurately recognize it as originating from a particular point in space.
HRTFs have wide ranging applications, from virtual surround sound in media and gaming, to hearing protection in loud noise environments, and hearing assistance for the hearing impaired. Particularly, in fields hearing protection and hearing assistance, the ability to record and reconstruct a particular user's HRTF presents several challenges as it must occur in real time. In the case of an application for hearing protection in high noise environments, heavy hearing protection hardware must be worn over the ears in the form of bulky headphones, thus, if microphones are placed on the outside of the headphones, the user will hear the outside world but will not receive accurate positional data because the HRTF is not being reconstructed. Similarly, in the case of hearing assistance for the hearing impaired, a microphone is similarly mounted external to the hearing aid, and any hearing aid device that fully blocks a user's ear canal will not accurately reproduce that user's HRTF.
Thus, there is a need for an apparatus and system for reconstructing a user's HRTF in accordance to the user's physical characteristics, in order to accurately relay positional sound information to the user in real time.